1. Field of the Invention
The present invention relates to a vertical pump which is to be installed in a water pumping well in a drainage plant or the like and in which a full-speed operation may be carried out regardless of the level of water.
2. Description of the Invention
A general-type vertical pump installed in a water pumping well or the like presents the lowest water level (the operable lowest water level) LWL where, even though the suction port is located in water, there are produced eddies so that water mixed with air is sucked. The lowest water level LWL is inherent in each pump. When a conventional vertical pump is operated before the water level reaches the lowest water level LWL, the pump inevitably produces vibration or noise. Accordingly, when such a general-type vertical pump is operated at a full speed (full-speed waiting operation) regardless of the water level in the water pumping well, causing the pump to stand ready for an unforeseen flood, there are instances where, while the water level is below the lowest water level LWL, the pump produces strong vibration and noise, causing the pump to be functionally disordered. In this connection, the conventional vertical pump of the general type is so arranged as not to carry out the full-speed waiting operation, and adopts an operation system in which the pump is operated only when the water level is higher than the lowest water level LWL and stops operating when the water level is lower than the lowest water level LWL.
Recently, the ground layer is decreased in water retention due to decrease in green zone and increase in pavement accompanied by the development of urbanization. It is therefore remarkable that the amount of water flowing into the water pumping well or the like is apt to be increased. Further, it often occurs that a great amount of water suddenly and unexpectedly flows into the water pumping well as is the case of a so-called flash flood. Accordingly, the water level in the water pumping well or the like varies in a short period of time. In the operation system using the conventional vertical pump of the general type as above-mentioned, it is difficult to properly control the operation starting and stopping timings of the vertical pump. This involves the likelihood that an abnormal rise in water level causes a flood or an abnormal descent of water level causes the pump to be functionally disordered.
In view of the foregoing, the Applicant has proposed a vertical pump capable of carrying out a stable full-speed waiting operation regardless of the water level which is either higher or lower than the lowest water level LWL, as disclosed by Japanese Patent Laid-Open Publication SHO 63/134897.
FIG. 7 shows the vertical pump disclosed by this Japanese Patent Laid-Open Publication SHO 63/134897. In this vertical pump, a suction pipe 603 having one end opened in the air, communicates with a suction casing 602 disposed at the forward (upstream) of a pump impeller. A suction valve 604 is disposed at the tip of the suction pipe 603. The suction valve 604 is adapted to be controlled as opened or closed by a signal from a water level detector 605. This vertical pump may carry out a full-speed waiting operation and controlled in the following manner.
In the case where the water level in a water pumping well P is raised from a position lower than the lowest water level LWL, the suction valve 604 is opened while the water level detector 605 detects that the water level does not reach the lowest water level LWL, and the suction valve 604 is closed when the water level detector 605 detects that the water level in the water pumping well P has reached the lowest water level LWL. According to such a control, until the water level reaches the lowest water level LWL, air is sucked from the suction pipe 603 to the impeller chamber 606 so that water is not pumped up. Thus, a so-called air operation at a full speed is smoothly continued. On the other hand, after the water level has reached the lowest water level LWL, the air suction from the suction pipe 603 to the impeller chamber 606 is stopped and water is pumped up. Thus, a normal water pumping operation is carried out.
In the case where the water level in the water pumping well P is lowered from a position higher than the lowest water level LWL, while the water level detector 605 detects that the water level does not reaches the lowest water level LWL, the suction valve 604 remains closed so that the water pumping operation is continued. On the other hand, when the water level detector 605 detects that the water level has reached the lowest water level LWL, the suction valve 604 is opened and air is sucked to the impeller chamber 606 through the suction pipe 603 so that water pumping is stopped and the operation is changed to the air operation.
As described in the foregoing, the vertical pump disclosed by this Japanese Patent Laid-Open Publication SHO 63/134897 is so arranged as to carry out such a full-speed waiting operation as to cope with sudden rise or descent of water level. It is therefore possible to prevent a flood from being caused by an abnormal rise in water level and to also prevent the pump from being functionally disordered due to an abnormal descent of water level.
However, this vertical pump presents the problem that the water level detector 605 is susceptible to an influence of water quality, dust or the like. Further, the suction valve 604 requires an external drive source and a sequence control unit for operating this drive source.
To solve the problems above-mentioned, the Applicant has proposed a vertical pump capable of carrying out a stable full-speed waiting operation, regardless of the water level which is higher or lower than the lowest water level LWL, without use of the water level detector and the suction valve. FIG. 8 shows such a vertical pump.
In the vertical pump shown in FIG. 8, a suction pipe 703 is made in a reverse U-shape and the lower open end 707 of the inside passage of the pipe 703 is located in a level identical with or in the vicinity of the lowest water level LWL. A turned portion 708 of the reverse U-shape suction pipe 703 is located in a position higher than the pumping head corresponding to a maximum negative pressure generated in the suction part of an impeller chamber 706 when an impeller 701 is operated at a full speed.
When the water level in a water pumping well P is raised from a position lower than the lowest water level LWL, the vertical pump in FIG. 8 is operated in the following manner. While the water level does not reach the lower open end 707, air is sucked from the lower open end 707 to the impeller chamber 706 through the suction pipe 703. Accordingly, no water is pumped up but a full-speed air operation is smoothly continued. When the water level is further raised and reaches the lowest water level LWL, the lower open end 707 is sealed with water. While the air remaining in a bellmouth 709, a suction casing 702 and the suction pipe 703 is sucked, the operation of the vertical pump is quickly changed to the water pumping operation. At the time when the air remaining in the suction casing 702 is perfectly sucked, a perfect water pumping operation starts. The turned portion 708 of the suction pipe 703 is located in a position higher than the pumping head corresponding to a maximum negative pressure generated in the impeller chamber 706. Accordingly, during the water pumping operation, water is pumped up in the hanging portion 710 of the suction pipe 703 to such a height as to be kept in equilibrium with a negative pressure in the impeller chamber 706. In this connection, no water is fed from the suction pipe 703 to the impeller chamber 706. Thus, water is pumped up through the suction casing 702 to achieve a smooth water pumping operation.
On the other hand, where the water level in the water pumping well P is lowered from a position higher than the lowest water level LWL during the full-speed operation of the vertical pump, the vertical pump is operated in the following manner. When the water level does not reach the lower open end 707, the lower open end 707 remains sealed with water so that the water pumping operation is continued. Meanwhile, water is pumped up in the hanging portion 710 of the suction pipe 703 to such a height as to be kept in equilibrium with a negative pressure of the impeller chamber 706. When the water level reaches the lowest water level LWL and the water sealing of the lower open end 707 of the suction pipe 703 is released, the water pumped in the hanging portion 710 falls and the suction pipe 703 is opened in the entire length thereof. Accordingly, air is sucked from the lower open end 707 to the impeller chamber 706 through the suction pipe 703. With such air suction, the water pumping operation in the suction casing 702 and the like is stopped, so that the operation of the vertical pump is quickly and smoothly changed to the air operation.
Thus, the vertical pump as shown in FIG. 8 may stand ready in a full-speed operation state for sudden rise or descent of the water level. It is therefore possible to prevent a flood from being caused by an abnormal rise in water level and to prevent the pump from being functionally disordered due to abnormal descent of water level. Further, it is not required for the vertical pump in FIG. 8 to use the water level detector 605 and the suction valve 604 discussed in connection with FIG. 7. Accordingly, the vertical pump in FIG. 8 eliminates the problems that the water level detector 605 is susceptible to an influence of water quality, dust or the like and that the control unit is somewhat complicated. Thus, the vertical pump may securely cope with an unexpected flood or the like.
In the vertical pump in FIG. 8, however, when the water level in the vicinity of the lowest water level LWL varies at a low speed or the water surface undulates in the vicinity of the lowest water level LWL, the lower open end 707 of the suction pipe 703 is repeatedly submerged under water or opened in the air in a short period of time. In such a case, there are instances where the open area or the open time of the lower open end 707 at the time when the lower open end 707 is opened in the air, is insufficient. This may cause a so-called hunting phenomenon where an insufficient air suction to the impeller chamber 706 through the suction pipe 703 is intermittently made in a short period of time. If such a hunting phenomenon occurs, the changeover from the air operation to the water pumping operation or from the water pumping operation to the air operation may not be smoothly carried out. This involves the likelihood that a force is exerted to the impeller 701 to cause the pump to be functionally disordered. Such a hunting phenomenon often occurs particularly when the water level in the water pumping well P is lowered from a position higher than the lowest water level LWL so that the water pumping operation is changed to the air operation. More specifically, the changeover from the air operation to the water pumping operation and vice versa during the full-speed waiting operation, is carried out based on the level of the lower open end 707 which is identical with or in the vicinity of the lowest water level LWL. Accordingly, if the open area or open time of the lower open end 707 is insufficient, it is not possible to assure air to be sucked to the suction casing 702 in such a sufficient amount as to quickly stop the water pumping operation which has been carried out up to that time.